1. Field of the Invention
This invention relates to a method of heat treating a preheated pulverous raw material consisting entirely of or containing at least a portion of lime prior to burning it in a rotary kiln. The invention relates to an improved method of at least partially calcining such pulverous, raw material and improved calcination plants for treating such raw material according to the improved method wherein heat is supplied before the material is subjected to any finishing calcination and/or any other heat treatment such as sintering in a rotary kiln. The invention also relates to a rotary kiln plant for burning such raw materials.
2. Description of the Prior Art
Calcination in the present context is to be understood as an expulsion of carbon dioxide from calcium carbonate according to the equation: EQU CaCo.sub.3 .fwdarw. CaO+ CO.sub.2
which is an endothermic process (i.e. a process which absorbs heat). When the raw material is cement raw meal, the aforesaid finishing heat treatment following the calcination is a sintering by which cement clinker is produced. Such sintering is an exothermic process in which heat is given off.
The heat necessary for carrying through the conversion of the cement raw meal to cement clinker is usually provided by burning fuel which together with combustion air is introduced into a combustion chamber and forms smoke gas. As a result, the energy contained in the fuel is released for heating the smoke gas to a high temperature. The hot smoke gas is then brought into contact with the raw meal to be heat treated. The heat is mainly used for preheating and calcining the raw meal, its sintering being as mentioned an exothermic process in practice, however, heat must be supplied in order to start the sintering.
Owing, among other things, to the presence of alkalis in the raw meal and the consequent drawbacks it is sometimes preferred to carry through the preheating and calcining of the raw meal by hot gas from one source of heat and the initiation of its sintering by hot gas from another source of heat.
In the case of calcination of cement raw meal it is desirable to carry through this process at a low temperature. However it is difficult to do that by means of smoke gases having a high temperature since there is then a great risk of excessive heating of the raw meal occurring locally and temporarily. Even excessive heating of a part of the raw meal for a short time may involve expulsion of alkali vapors or the production of melts which may give rise to cakings. Excessive heating of the raw meal at the calcination stage may also prevent chemical reactions intended to take place at a later stage of the whole process for the manufacture of cement clinker. For example, a clinker mineral formation at the stage of the total process at which the calcination is to take place will involve a disadvantageous development of the whole process.
In a related development, my commonly assigned U.S. patent application Ser. No. 423,436, filed Dec. 10, 1973, now U.S. Pat. No. 3,955,995 is directed to calcination of pulverous material by mixing preheated raw material intimately with a fuel capable of producing a combustible gas upon contacting the raw material, providing a gas capable of supporting combustion of the combustible gas thus produced to at least partially calcine the raw material, and separating the treated raw material from the stream of the gases. In one embodiment a stream of oxygen-containing gas is passed upwards through the center of a tubular calcination chamber and the preheated raw material and fuel are intimately mixed in a duct which discharges a suspension of the fuel gas/raw material into the bottom portion of the calcination chamber and into contact with the oxygen-containing gas stream. As a result some eddys are formed between the central gas stream and the chamber wall with simultaneous burning of the fuel gas and calcination of the raw material at a low temperature.
U.S. Pat. No. 3,203,681 to Rosa, et al. relates to a process wherein heat for carrying through the calcination of preheated cement raw meal derives from hot gases having a temperature higher than the calcination temperature. The gases are produced in a separate chamber and are passed upwardly in a riser column in which the raw material is suspended and entrained by the gases thus produced. U.S. Pat. No. 3,452,968 to Shimizu, et al. relates to a process for roasting fine ore wherein preheated raw meal and fuel are ejected individually into a rotating flow of gas ascending upwardly in a calcining chamber. Combustion and the roasting reaction are thus caused in the violently diffusing turbulent flow. U.S. Pat. No. 2,776,132 to Pyzel relates to a process wherein a fluidized bed of hydraulic cement is maintained by charging air into the bottom portion thereof. Raw uncalcined materials in powdered form are charged into the bed along with an amount of fuel sufficient to maintain the bed at a temperature high enough to bring about calcination of raw uncalcined carbonate materials and the formation of cement from the calcined raw materials. U.S. Pat. No. 2,874,950 to Pyzel relates to a process wherein unpreheated cement raw meal is maintained in a fluidized state within a reaction zone by an upward flow of oxygen containing gas. Combustible fuel is charged into the reaction mass in order to maintain the temperature necessary for the heat treatment which consists of calcination and sintering to be carried out within the reaction zone. U.S. Pat. No. 3,013,786 to Pyzel relates to a process wherein preheated feed materials are introduced into a calcining furnace equipped with a burner at one end. Air and fuel are supplied to the calcining furnace through separate pipe means. Combustion of fuel and air takes place in the calcining furnace and the powdered feed material are charged into the combustion gases so generated and are heated to calcining temperatures while carried in suspension in the gases.
According to my invention, a raw material consisting entirely of, or at least containing a portion of lime is at least partially calcined substantially isothermally (i.e. constant temperature conditions) -- prior to being burned -- at relatively low temperatures while substantially eliminating the disadvantages of the presently known systems particularly by simplifying the process by feeding fuel and preheated raw material separately into the calcination chamber and by thus providing improved contact between the raw material, the fuel and the combustion gas.